1. Field of the Invention
The present invention relates to a piezoelectric actuator, and more particularly, to a slit type piezoelectric actuator.
2. Description of Related Art
A piezoelectric actuator is a transducer which converts electric energy to mechanical energy by using the electrostrictive effect of a piezoelectric element.
More specifically, an electrostrictive effect element utilizes the strain generated in the piezoelectric element which exhibits a large electrostrictive effect, such as a lead magnesium niobate-lead titanate ceramic and a lead zirconate titanate ceramic, when an electrode made of a metal or the like is formed on each of two facing surfaces of the piezoelectric element, and a drive voltage is applied between the electrodes.
In the above-mentioned electrostrictive effect, the strain that is generated in the direction parallel to the applied electric field (longitudinal electrostrictive effect) is ordinarily about twice as large as the strain generated in the direction perpendicular to the electric field (lateral electrostrictive effect), so that it is advantageous to utilize the former, and the conversion efficiency from the electrical energy to the mechanical energy is also high.
In an piezoelectric actuator which utilizes the longitudinal electrostrictive effect (referred to as an actuator hereinafter), the amount of displacement per unit length of the actuator is substantially proportional to the intensity of the applied electric field.
Namely, in order to obtain a large amount of displacement it is necessary either to apply a high voltage between the opposing electrodes or to reduce the distance between the facing electrodes.
However, there is required a large-sized and an expensive power unit in order to apply a high voltage, and hazard in handling is also increased, so that it is more desirable to reduce the distance between the facing electrodes.
Heretofore, there is proposed a piezoelectric actuator having the laminated ceramic capacitor type structure to realize the actuator with smaller distance between the facing electrodes.
The structure of the actuator with the above-mentioned type is shown in FIG. 1A and FIG. 1B.
FIG. 1A is a cross-sectional view of the actuator taken in a plane parallel to the lamination direction of the element. In addition, FIG. 1B is a projected view of the actuator in the lamination direction thereof.
As shown in FIG. 1A, this actuator consists of a ceramic part where internal electrodes are provided in its interior with a specified spacing, and a ceramic part where no internal electrode is involved.
The ceramic part which has the internal electrodes in the interior is ordinarily called an active layer 1, and it is the part which contributes to the operation as an element by generating the longitudinal electrostrictive effect in the ceramic when a voltage is applied between the internal electrodes.
In the active layer 1, internal electrodes 3a and 3b are formed with a specified spacing in the interior of ceramic 2, alternately connected for every other layer to external electrodes 4a and 4b that are provided on the side face of the actuator, and the adjacent internal electrodes are mutually forming facing electrodes with a ceramic layer in between.
The distance between the internal electrodes can be made to have a minimum value of about 10 .mu.m by means of the ordinary laminated ceramic capacitor technology.
On the other hand, the part which does not have in its interior the internal electrodes is ordinarily called an inactive layer 5, and it is the part which does not contribute to the operation of the actuator without showing the electrostrictive effect because of the absence of the internal electrodes for applying electric field therebetween. However, this part is necessary to prevent the fluctuation in the components of ceramic 2 of the active layer 1 at the time of high temperature heat treatment during the manufacturing process that will be described later.
By giving the above-mentioned laminated ceramic capacitor structure to the element it is possible to reduce the distance between the internal electrodes of the active layer 1. Therefore, it becomes possible to apply a high electric field to the electrostrictive material even with a low voltage, thereby realizing an element utilizing the longitudinal electrostrictive effect that can be operated at low voltages.
Now, in an actuator with the aforementioned structure the area where the internal electrodes overlap (the portion of the innermost rectangle in FIG. 1B) is small compared with the cross-sectional area of the actuator.
Because of this, when a large strain is generated in the part where the internal electrodes overlap by the application of a voltage to the actuator, there occurs a strong stress concentration in the boundary portion 6 between the part where the internal electrodes overlap and the part where they do not, and there may arise cases in which the actuator is mechanically broken if the applied voltage is high enough.
In order to improve such a defect of the conventional actuator, an actuator is proposed in which grooves (referred to as slits hereinafter) parallel to the internal electrodes are provided on the side face of the laminated ceramic capacitor type piezoelectric actuator.
The actuator with this structure is disclosed in Japanese Patent Laid Open No. 58-196077, and its structure is shown in FIG. 2A and FIG. 2B. An actuator of this structure will be called a slit type actuator hereinafter.
FIG. 2A shows a sectional view in a plane parallel to the lamination direction of the slit type actuator, and FIG. 2B shows the side face of the slit type actuator where external electrodes are not formed.
In this slit type actuator, it is possible to prevent mechanical breakdown of the slit type actuator by dispersing and relaxing the concentration of stress at the time of generation of a displacement in the slit type actuator, by providing a plurality of slits that encircle the side face of the actuator in the parts where no overlap of the internal electrodes exists and hence no strain arises (namely, the portion of the ceramic between the external electrodes 4a and 4b, and the internal electrodes 3b and 3a).
As a result, it is possible to obtain a larger displacement since a voltage higher than for the conventional actuator becomes applicable.
Next, the manufacturing process of the slit type actuator will be described.
First, using the powder of a material that exhibits the electrostrictive effect, such as a lead magnesium niobate-lead titanate ceramic or a lead zirconate titanate ceramic, as the starting material, an organic solvent, a binder, and a plasticizer are added, and a slurry is prepared by stirring and mixing these ingredients.
Next, a ceramic green sheet is prepared from the slurry by doctor blade method or the like.
After drying the green sheet, a paste for internal electrodes having the powder of a silver-palladium alloy as the principal ingredient and a paste for slit forming material having carbon as the principal ingredient are printed over the green sheet by screen printing or the like.
Next, a ceramic laminated body is obtained by integration through stacking and thermocompression bonding.
By treating the ceramic laminated body at 600.degree. C. in the air, the binder and the slit forming material are thermally decomposed and dispersed, and there are formed vacancies for the slits.
Subsequently, a ceramic sintered body having in its interior the vacancies for the slits is obtained by calcining the ceramic laminated body at a high temperature of about 1100.degree. C. Then, the sintered body is cut, and after firing a silver paste for external electrodes on the two side faces where the internal electrodes are exposed, lead wires 8 for applying external voltage are connected, completing the slit type actuator.
It is to be noted that in order to take out the displacement of the actuator to the outside in the actual use of the actuator, it is general to bond the top and the bottom end faces of the actuator to metallic jigs 9 using an adhesive 10 or the like to transfer the displacement in the actuator to the metallic jig 9, as shown in FIG. 3.
The prior art slit type actuator sometimes generates the following inconvenience in the course of its manufacturing process.
FIG. 4 shows the state of affairs in the midst of the process for manufacturing a conventional slit type actuator, illustrating a sectional view of lamination of green sheets 13 with the patterns for the internal electrode paste 11 and for the slit forming material 12 being printed. It should be noted that FIG. 4 is drawn with the inactive layer part omitted.
In FIG. 4, the portion of the green sheet indicated as A has no internal electrodes printed, and there are laminated green sheets on which is printed the pattern for the slit forming member 12 alone on every fifth layer.
In contrast, the portion of the figure indicated as B, the pattern for the internal electrode paste alone is printed, and a large number of these layers are laminated.
In the figure, the alternate long and short dash line drawn in the portion where the slit forming materials are laminated (the portion indicated as A) shows the cutting position for segmenting the slit type actuator after sintering the ceramic laminated body.
FIG. 5 is a sectional view of the ceramic sintered body obtained by thermocompression bonding and calcining the ceramic laminated body. In addition to the integration of the whole system, the slit forming material is thermally decomposed and dispersed to form the vacancies 14 for the slits.
In FIG. 5, in the portion indicated as A, the number of layers with printed pattern is small so that the degree of application of the pressure at the time of the thermocompression bonding is weaker compared with the case for the part indicated as B.
Because of this, the amount of contraction at the time of sintering is greater for the portion A than for the portion B, generating a non-uniform contraction as a whole and making the generation of cracks in the portion A easier.
When such cracks 15 are generated, the mechanical strength of the slit type actuator segmented from the ceramic sintered body is reduced so that the element tends to be destroyed when a bending force is applied.
Furthermore, when large and deep cracks that reach even to the region between the facing internal electrodes are generated, the application of a voltage for the purpose of obtaining a displacement may lead to the breakdown of the actuator due to a discharge generated between the internal electrodes.
For this reason, with the conventional slit type actuator it is difficult to take out a large displacement by the application of a high voltage, so that the voltage to be applied had to be limited to low values.
Next, problems that can be generated in the use of the slit type actuator will be described.
As described in the above, to take out the displacement of the actuator to the outside it is ordinary, in general, to use the actuator by bonding its top and bottom end faces to metallic jigs 9 as shown in FIG. 3.
In that case, as the adhesive 10, use is made of a resin with high young's modulus so as not to absorb the displacement of the actuator, and it is general to use a thermosetting resin.
Accordingly, the bonding of the metallic jigs 9 to the actuator is carried out by cooling the heated state at 150.degree. to 200.degree. C. to the ordinary temperatures.
In this operation, the amount of contraction at cooling in the bonding process is larger for the metallic jig because the coefficient of thermal expansion of the metallic jig 9 is greater than the coefficient of expansion of the actuator.
Because of this, compressive forces as shown by the arrows in FIG. 3 is generated in the end face of the actuator, and as a result, the inactive layer 5 is deformed as shown by the broken line in the figure.
Further, the deformation of the actuator when a voltage is applied to the actuator in the above-mentioned actuator will be considered.
In FIG. 3, when a voltage is applied to the actuator, the active layer 1 is elongated in the direction of lamination due to the longitudinal electrostrictive effect and contracts in the direction perpendicular to the direction of lamination due to the lateral electrostrictive effect.
However, there will be generated no deformation in the inactive layer 5 so that the active layer 1 is urged to undergo a deformation as shown by the alternate long and short dash line in FIG. 3 (note that the elongation due to the longitudinal electrostrictive effect is not shown in the Figure).
Therefore as the combined effect of the two kinds of deformation there is generated a large tensile stress at the interface of the active layer 1 and the inactive layer 5, that is, at the part of the internal electrode 3a of the outermost layer.
However, the strength of the actuator against tension is weaker at the interface between the ceramic and the internal electrode than in the ceramic itself which is the electrostrictive material.
Accordingly, when a high voltage is applied to the actuator of this structure for the purpose of taking out a large displacement, mechanical breakdown tends to be generated at the interface of the internal electrode 3c of the outermost layer.
As is naturally expected, similar situation occurs also at the internal electrode of the outermost layer on the opposite side (namely, on the side of the bottom end of the actuator).
Although the description in the above has been made with reference to an ordinary actuator which is not of the slit type, similar phenomenon also takes place in the slit type actuator.
For this reason, in the conventional slit type element it is necessary to limit the voltage to be applied to lower values in order to prevent mechanical breakdown at the interface of the internal electrode of the outermost layer.
Summarizing the above, in the conventional slit type actuator, cracks tend to be generated in the course of the manufacturing process of the actuator, and moreover, a large tensile stress acts on the interface of the internal electrode of the outermost layer at the time of its use.
Because of this, the conventional slit type actuator has a defect in that it is necessary in its use to limit the applied voltage to a low level in order to prevent the dielectric breakdown between the internal electrodes and the external electrodes, and to prevent the mechanical breakdown of the actuator, so that it is difficult to take out a large displacement by the application of a high voltage.